Preheat oven to 450 degrees. Spray a 9 inch pie dish with non-stick cooking spray. Spread the pie crust out evenly in the pan and press gently to bottom and sides. Flute the top of the pie crust.
Using a fork, poke holes about every inch in the bottom of the crust and also, the sides. Bake the crust in the preheated oven for 10-12 minutes, or until golden brown. Set aside to cool and turn off the oven.
In a medium saucepan, combine the sugar, flour, salt, egg yolks, and cocoa, stirring to mix well. Add enough of the evaporated milk to make a paste, stirring to check consistancy, and then pour the remainder of the canned milk into a two cup measuring cup.
Add enough milk to make two cups (approximately 3/4 cup).
Add the two cups of the milk to the saucepan along with the butter. Cook on medium high heat, stirring constantly until you get a consistancy that''s like pudding or a little bit thicker.
Allow the chocolate filling to cool completely before pouring into the pie crust. Top with merangue and bake at 350 until peaks are a light golden brown, or top with whipped topping of your choice. If you bake the merange, allow the pie to cool completely before cutting. If you top with whipped topping, refrigerate pie for about 30 minutes before adding the topping.